Version analysis of SHPPT - Elite on the 6502

This code appears in the following versions (click to see it in the source code):

Cassette, Disc (flight), Disc (docked), 6502 Second Processor, Master and Electron

Code variations between these versions are shown below.

       Name: SHPPT
       Type: Subroutine
   Category: Drawing ships

Code variation 1 of 13A variation in the comments only

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Cassette, Flight, 6502SP, Master, Electron
Docked
    Summary: Draw a distant ship as a point rather than a full wireframe
    Summary: Draw a distant ship as a point in the middle of the screen



.SHPPT

Code variation 2 of 13Related to the Master version

When drawing a distant ship as a dot, the cassette, disc and 6502SP versions erase the entire on-screen ship before redrawing it, while the Master version erases and redraws each ship one line at a time.

See below for more variations related to this code.

This variation is blank in the Master version.

Cassette, Flight, Docked, 6502SP, Electron
 JSR EE51               \ Call EE51 to remove the ship's wireframe from the
                        \ screen, if there is one

Code variation 3 of 13Specific to an individual platform

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Cassette, Flight, 6502SP, Master, Electron
Docked
 JSR PROJ               \ Project the ship onto the screen, returning:
                        \
                        \   * K3(1 0) = the screen x-coordinate
                        \   * K4(1 0) = the screen y-coordinate
                        \   * A = K4+1

 ORA K3+1               \ If either of the high bytes of the screen coordinates
 BNE nono               \ are non-zero, jump to nono as the ship is off-screen

 LDA K4                 \ Set A = the y-coordinate of the dot
 LDA #Y                 \ Set A = the y-coordinate of a dot halfway down the
                        \ screen

Code variation 4 of 13A variation in the comments only

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Cassette, Flight, 6502SP, Master, Electron
Docked
 CMP #Y*2-2             \ If the y-coordinate is bigger than the y-coordinate of
 BCS nono               \ the bottom of the screen, jump to nono as the ship's
                        \ dot is off the bottom of the space view
 CMP #Y*2-2             \ If the y-coordinate is bigger than the y-coordinate of
 BCS nono               \ the bottom of the screen, jump to nono as the ship's
                        \ dot is off the bottom of the space view. This will
                        \ never happen, but this code is copied from the flight
                        \ code, where A can contain any y-coordinate

Code variation 5 of 13Related to the Master version

See variation 2 above for details.

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Cassette, Flight, Docked, 6502SP, Electron
Master
 LDY #2                 \ Call Shpt with Y = 2 to set up bytes 1-4 in the ship
 JSR Shpt               \ lines space, aborting the call to LL9 if the dot is
                        \ off the side of the screen. This call sets up the
                        \ first row of the dot (i.e. a four-pixel dash)

 LDY #6                 \ Set Y to 6 for the next call to Shpt
 JSR Shpt               \ Call Shpt to draw a horizontal four-pixel dash for the
                        \ first row of the dot (i.e. a four-pixel dash)

Code variation 6 of 13Specific to an individual platform

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Cassette, Flight, 6502SP, Electron
Docked
Master
 LDA K4                 \ Set A = y-coordinate of dot + 1 (so this is the second
 ADC #1                 \ row of the two-pixel high dot)
                        \
                        \ The addition works as the Shpt routine clears the C
                        \ flag
 LDA #Y                 \ Set A = #Y + 1 (so this is the second row of the
 ADC #1                 \ two-pixel high dot halfway down the screen)
                        \
                        \ The addition works as the Shpt routine clears the C
                        \ flag
 LDA K4                 \ Set A = y-coordinate of dot + 1 (so this is the second
 CLC                    \ row of the two-pixel high dot)
 ADC #1

Code variation 7 of 13A variation in the comments only

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Cassette, Flight, Docked, 6502SP, Electron
Master
 JSR Shpt               \ Call Shpt with Y = 6 to set up bytes 5-8 in the ship
                        \ lines space, aborting the call to LL9 if the dot is
                        \ off the side of the screen. This call sets up the
                        \ second row of the dot (i.e. another four-pixel dash,
                        \ on the row below the first one)
 JSR Shpt               \ Call Shpt to draw a horizontal four-pixel dash for the
                        \ second row of the dot (i.e. a four-pixel dash)

 LDA #%00001000         \ Set bit 3 of the ship's byte #31 to record that we
 ORA XX1+31             \ have now drawn something on-screen for this ship
 STA XX1+31

Code variation 8 of 13Related to an advanced feature

Ships in the advanced versions each have their own colour, including when they are shown as points.

This variation is blank in the Master version.

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Cassette, Flight, Docked, Electron
6502SP
 LDA #8                 \ Set A = 8 so when we call LL18+2 next, byte #0 of the
                        \ heap gets set to 8, for the 8 bytes we just stuck on
                        \ the heap
 LDA #9                 \ Set A = 9 so when we call LL18+2 next, byte #0 of the
                        \ heap gets set to 9, to cover the 9 bytes we just stuck
                        \ on the heap

Code variation 9 of 13Related to the Master version

See variation 2 above for details.

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Cassette, Flight, Docked, 6502SP, Electron
Master
 JMP LL81+2             \ Call LL81+2 to draw the ship's dot, returning from the
                        \ subroutine using a tail call

 PLA                    \ Pull the return address from the stack, so the RTS
 PLA                    \ below actually returns from the subroutine that called
                        \ LL9 (as we called SHPPT from LL9 with a JMP)
 JMP LSCLR              \ Jump to LSCLR to draw any remaining lines that are
                        \ still in the ship line heap and return from the
                        \ subroutine using a tail call

.nono

 LDA #%11110111         \ Clear bit 3 of the ship's byte #31 to record that
 AND XX1+31             \ nothing is being drawn on-screen for this ship
 STA XX1+31

Code variation 10 of 13Related to the Master version

See variation 2 above for details.

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Cassette, Flight, Docked, 6502SP, Electron
Master
 RTS                    \ Return from the subroutine
 JMP LSCLR              \ Jump to LSCLR to draw any remaining lines that are
                        \ still in the ship line heap and return from the
                        \ subroutine using a tail call

.Shpt

Code variation 11 of 13Related to the Master version

See variation 2 above for details.

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Cassette, Flight, Docked, 6502SP, Electron
Master
                        \ This routine sets up four bytes in the ship line heap,
                        \ from byte Y-1 to byte Y+2. If the ship's screen point
                        \ turns out to be off-screen, then this routine aborts
                        \ the entire call to LL9, exiting via nono. The four
                        \ bytes define a horizontal four-pixel dash, for either
                        \ the top or the bottom of the ship's dot

 STA (XX19),Y           \ Store A in byte Y of the ship line heap (i.e. Y1)

 INY                    \ Store A in byte Y+2 of the ship line heap (i.e. Y2)
 INY
 STA (XX19),Y
                        \ This routine draws a horizontal four-pixel dash, for
                        \ either the top or the bottom of the ship's dot

 STA Y1                 \ Store A in both y-coordinates, as this is a horizontal
 STA Y2                 \ dash at y-coordinate A

Code variation 12 of 13Specific to an individual platform

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Cassette, Flight, 6502SP, Master, Electron
Docked
 LDA K3                 \ Set A = screen x-coordinate of the ship dot
 LDA #X                 \ Set A = x-coordinate of the middle of the screen

Code variation 13 of 13Related to the Master version

The Master implements flicker-free ship drawing using the LSPUT routine, which is used both for drawing wireframes and for drawing distant ships as dots.

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Cassette, Flight, Docked, 6502SP, Electron
Master
 DEY                    \ Store A in byte Y+1 of the ship line heap (i.e. X2)
 STA (XX19),Y

 ADC #3                 \ Set A = screen x-coordinate of the ship dot + 3

 BCS nono-2             \ If the addition pushed the dot off the right side of
                        \ the screen, jump to nono-2 to return from the parent
                        \ subroutine early (i.e. LL9). This works because we
                        \ called Shpt from above with a JSR, so nono-2 removes
                        \ that return address from the stack, leaving the next
                        \ return address exposed. LL9 called SHPPT with a JMP,
                        \ so the next return address is the one that was put on
                        \ the stack by the original call to LL9. So the RTS in
                        \ nono will actually return us from the original call
                        \ to LL9, thus aborting the entire drawing process

 DEY                    \ Store A in byte Y-1 of the ship line heap (i.e. X1)
 DEY
 STA (XX19),Y

 RTS                    \ Return from the subroutine
 STA X1                 \ Store the x-coordinate of the ship dot in X1, as this
                        \ is where the dash starts

 CLC                    \ Set A = screen x-coordinate of the ship dot + 3
 ADC #3

 BCC P%+4               \ If the addition overflowed, set A = 255, the
 LDA #255               \ x-coordinate of the right edge of the screen

 STA X2                 \ Store the x-coordinate of the ship dot in X1, as this
                        \ is where the dash starts

 JMP LSPUT              \ Draw this edge using flicker-free animation, by first
                        \ drawing the ship's new line and then erasing the
                        \ corresponding old line from the screen, and return
                        \ from the subroutine using a tail call